Leather-like sheets consisting of ultra-fine fibers and an elastomer have excellent features unavailable in natural leather and are widely used in various applications. As a method generally employed for producing such a leather-like sheet, a fiber sheet is impregnated with an elastomer solution of a polyurethane or the like, and the impregnated fiber sheet is immersed in water or an organic solvent aqueous solution, to wet-coagulate the elastomer.
However, since the polyurethane must be used in a large amount for obtaining, for example, strength and size stability, the raw material cost of the polyurethane, complicated production process and the like make the leather-like sheet expensive. Furthermore, a higher elastomer content is likely to cause rubber-like hand, making it difficult to obtain a compactness similar to that of natural leather. Moreover, for the necessity of impregnation with the polyurethane, a water miscible organic solvent such as N,N′-dimethylformamide is generally used, though such organic solvents are not generally preferable in view of working environment.
Furthermore, in recent years, recyclability is respected for the purpose of protecting the environment, resources and the like, and in this connection, for example, polyester decomposing and recovering methods (for example, WO 01/30729) and polyurethane decomposing methods (for example, JP 2001-348457 A) are studied. However, these methods are mainly applied to a material consisting of a single component, and it is difficult to apply the methods to a composite material having fibers and an elastomer such as a polyurethane inseparably integrated as described above, since different decomposing methods are needed. So, separation into respective components is necessary, but in general the separation cost is high while perfect separation into respective components is also difficult.
Furthermore, it is reported that, for example, a polyurethane is yellowed by NOx gas or the like, and it is difficult to obtain a white suede-like sheet.
Therefore, a leather-like sheet containing less or substantially no elastomer such as a polyurethane is desired.
To solve these problems, it is an effective means to enhance the strength of the nonwoven fabric per se. Several means for enhancing the strength of the nonwoven fabric per se have been studied so far. For example, disclosed is a nonwoven fabric to be used as a leather-like sheet, consisting of fiber bundles and single fibers, obtained by using self-bonding fibers such as cellulose fibers for forming self-bonded bundles, treating them by such a means as needle punching to form a sheet, and jetting a high speed fluid flow to the sheet, to entangle the bundles with each other, to entangle the bundles with the single fibers and to entangle the single fibers with each other (for example, JP 52-12902 A). However, if bundles are bonded by such a method, there arise such problems that when the nonwoven fabric is dyed, color irregularity occurs and that the surface appearance and hand become poor. There is also a further other problem that since the high speed fluid flow causes the considerable portions of the self-bonded ultra-fine fibers to be debonded and entangled, irregular debonding occurs due to irregular treatment, making the control of debonding difficult.
On the other hand, proposed are various methods in which needle punching is followed by hydro-entanglement to improve entanglement (for example, JP 1-18178 B and JP 5-78986 A). These methods are respectively effective as a means for enhancing the entangling efficiency of hydro-entanglement. However, we the inventors found that even if needle punching and hydro-entanglement are merely combined, it is difficult to obtain a nonwoven fabric lowered in polyurethane content and still having satisfactory physical properties and quality maintained.
Furthermore, as a means different from the above-mentioned ones, it is disclosed that if polyester fibers with a low modulus and heat shrinkable polyester fibers are needle-punched, subsequently heat-treated and hot-pressed, a base sheet for a leather-like sheet having sufficient performance even without being impregnated with a polyurethane can be obtained (for example, JP 7-62301 B). However, we the inventors found that when the nonwoven fabric obtained like this was dyed, for example, using a jet dyeing machine, it was often broken by massaging and the like.